What work is currently being done to improve the flux calibration accuracy for echelle modes?
Update:
All the issues mentioned below have now been calibrated. Users are
advised to download the data via the OTFR system, if the data on hand
were retrieved before 2006-Nov-03. Please refer to the
October/November 2006 Newsletter for more information.
Three different issues are currently under investigation, namely:
- Shift of the echelle blaze function with time and location on the detector - The passage of time on orbit and monthly offsetting of spectra on the MAMA detectors cause the echelle blaze function to shift with respect to both wavelength and detector pixels, leading to sawtooth flux calibration errors as large as 15%. The calstis task "x1d" now has a "blazesh" parameter that attempts to compensate for these effects by manually shifting the flux calibration curve. For primary central wavelength settings, the STScI pipeline uses the observation time and spectrum location to crudely estimate the blaze shift, recording the estimate in the BLZSHIFT keyword in the _x1d extension header. In addition, the user can run "x1d" with his/her own preferred value of the "blazesh" parameter. Starting early in Cycle 11, monthly offsetting for echelle modes was turned off to minimize these types of calibration problems.
- Scattered-light subtraction - The STScI pipeline now includes a 2-D scattered light removal algorithm for the STIS echelle modes (see STIS ISR 2002-01), which reduces the associated flux uncertainties to this effect to less than 2%. We are currently working on redetermining the echelle flux calibration so that it is self-consistent with the 2-D scattered light algorithm.
- Time dependence of the sensitivity - Sensitivity-monitor data for the echelle modes are currently under investigation. The trends appear to be similar to the first-order gratings (see STIS ISR 2001-01R) with a general linear decrease by 1-2% per year, depending on wavelength. Software changes and/or calibration reference file updates are being developed.